1. Field of the Invention
This invention relates generally to methods and devices for optical frequency conversion and more particularly to a method for first-order optical frequency conversion by quasi-phase-matching (QPM), and an inorganic, thin film waveguide device for generating first-order, quasi-phase-matched, frequency converted light.
2. Description of the Related Art
It is particularly useful to be able to generate shorter wavelength optical radiation from a longer wavelength optical source by changing the frequency of the source radiation. Frequency mixing or frequency doubling, also referred to as second harmonic generation (SHG), have generally been the techniques used to accomplish this. Devices which are capable of frequency conversion and which are made from bulk inorganic nonlinear optical materials are well known in the art.
A consistent obstacle in the continued advancement of frequency conversion devices is more efficient conversion of fundamental input optical radiation to frequency converted output light. In general, high efficiency frequency conversion requires consideration of the following: the desirability of optical materials which are strongly nonlinear, i.e., having relatively high second-order nonlinear optical coefficients; availability of high source intensities at the input frequency because the nonlinear coefficients for currently available nonlinear materials are small; the ability to phase-match the fundamental and second harmonic waves in a waveguide; i.e., equalize and match their phase velocities thus allowing energy to be continuously transferred from the fundamental source or input wave to the second harmonic output wave. Other desirable conditions include low optical absorption and scattering by the waveguide materials at both the input and converted frequencies; good mechanical properties of the substrate; environmental stability; and, compatibility with standard optical fabrication technology.
At the time of this invention, more efficient second harmonic generation in nonlinear optical waveguides formed in bulk single crystals had been demonstrated using third-order quasi-phase-matching. This approach offered several advantages over the more conventional nonwaveguide methods of frequency conversion using birefringence to achieve phase matching. The ability of a waveguide to confine light over long propagation distances allows for high intensity input light to propagate over a considerable interaction length; that length comprising the active region of the propagation medium in which quasi-phase-matching occurs, contributing to the high overall efficiency of quasi-phase-matched frequency conversion. High quality nonlinear single crystals, however, are expensive. Furthermore, bulk single crystal substrates typically serve only the limited function of a substrate, thereby lacking alternate utility that semiconductor devices, for example, might provide. Another disadvantage of using bulk single crystal materials is the lower limit waveguide thickness achievable with waveguides formed as modified surface structures in bulk materials. The consequence of thick waveguides includes larger waveguide cross-sectional areas and thus lower waveguide mode field intensities which result in lower conversion efficiency. Degradation of the nonlinear optical coefficient has also been reported during the process of waveguide formation in bulk single crystals. These material limitations, however, must be considered in conjunction with the advantages of using quasi-phase-matching to achieve optical frequency conversion.
Quasi-phase-matching provides a flexible approach to the phase matching problem in that there is no need to compensate for material dispersion; that is, the variation in the index of refraction in the material as a function of wavelength, as with birefringence phase-matching. QPM also eliminates the need to rely on waveguide dispersion for frequency conversion, which requires stringent waveguide tolerancing. Quasi-phase-matching allows the free choice of crystalline orientation and propagation direction for a given nonlinear material so that the highest nonlinear optical coefficient of the material can be utilized. Furthermore, quasi-phase-matching can be achieved for any arbitrary wavelength.
Quasi-phase-matching frequency conversion efficiency is closely related to the order number of the QPM structure. QPM is accomplished when the direction of the polar axis of the ferroelectric, nonlinear crystalline material is periodically reversed by 180 degrees along the propagation direction of the input optical radiation. The sign of the nonlinear optical coefficient, effectively, the sign of the second order dielectric susceptibility component, likewise alternates as the crystal polar axis periodically reverses in direction This allows the transfer of energy from the fundamental frequency to the converted frequency in a monotonically increasing way provided that the half-period of the alternating, nonlinear dielectric susceptibility component is an odd integral multiple of the coherence length. The coherence length is the propagation distance in the nonlinear medium over which a phase difference of .pi. is established between input frequency waves and frequency converted waves in the waveguide. Typical materials exhibit a coherence length of a few microns. The odd integer multiple of the coherence length, which characterizes the periodicity of the active region of the QPM structure, that is, the grating period of the sign-alternating nonlinear optical coefficient, is referred to as the order number, m, of the QPM structure. In the nondepletion limit, the efficiency of quasi-phase-matched frequency conversion varies as the inverse of the square of the order number; i.e., 1/m.sup.2.
First-order (m=1) QPM thus results in the most efficient frequency conversion. To date, the majority of work involving quasi-phase-matching in nonlinear, inorganic waveguides has dealt with waveguide formation in bulk, nonlinear, single crystals. Up to the time of the instant invention, the most efficient frequency conversion by quasi-phase-matching using surface modified, bulk single crystal waveguides has been limited to third-order OPM.
Lim et al. reported the generation of blue light at 410 nm by continuous-wave frequency-doubling in a periodically poled lithium niobate channel waveguide, using the d.sub.33 nonlinear coefficient, in Electronics Letters, Vol. 25, No. 11 (1989). More recently, in Appl. Phys. Lett. Vol 58, No. 24 (Jun. 1991), Mizuuchi et al. reported a third-order QPM-SHG device in LiTaO.sub.3 with a low-loss proton-exchanged waveguide.
Alternatively, Somekh and Yariv theoretically proposed periodically modulating the optical nonlinear coefficient of a thin film, dielectric, single crystal waveguide to achieve apparent first-order phase-matched optical frequency conversion as far back as 1972. They speculated that practical implementation of their work could be achieved in a thin film waveguide by sputter-filling ion-milled grooves in the waveguide with a polycrystalline form of the film material, thus rendering the nonlinear optical coefficient null in the filled areas. The overall value of the nonlinear optical coefficient in the propagation medium, however, would effectively be reduced because no energy conversion to the second harmonic wave would occur over every half-period of the periodically poled waveguide; that is, in the filled regions. Although they apparently devised a first-order quasi-phase-matched optical frequency conversion scheme, the anticipated efficiency of the speculative structure was still a factor of four less than true first-order quasi-phase-matched frequency conversion. The actual efficiency of their method, had such a device ever been built, is unknown.
In summary, existing optical frequency converters using quasi-phase-matching are materially and technically limited, while formerly proposed first-order quasi-phase-matched frequency conversion means failed, in principle, to achieve the potential of true first-order quasi-phase-matching.